1. Field of the Invention
The present invention relates generally to the field of surgical replacement of a diseased or damaged bone joint of a warm blooded mammal and specifically to the fixation of a prosthesis device in a human.
2. Background Art
An improved process and apparatus for orthopedic implantation of a prosthesis in a warm blooded mammals disclosed by Park in U.S. Pat. No. 4,266,303 is entitled "Method of Orthopedic Implantation and Implant Product". This patent described at some detail implant techniques and developments, and is hereby specifically incorporated herein, along with each of the other specifically identified patents, as part of the written description of the present invention. The Park patent disclosed a metal implant prosthesis provided with a uniform thin polymer bone cement coating on substantially the entire stem or surface portion of the prosthesis that is to be affixed to the bone. The preformed acrylic polymer bone cement coating is cured or polymerized under controlled conditions of temperature, pressure and environment to achieve desired optimum mechanical properties prior to being implanted. The preformed polymeric coating is selected to be chemically compatible with the bone cement to be used during the surgical implant procedure to insure the effecting of a strong chemical bond therebetween and greatly enhance the structural connection of the prosthesis to the bone.
U.S. Pat. No. 4,012,796 to Weisman et al. discloses an "Interpositioning Collar for Prosthetic Bone Insert". The separate collar or flange is formed by a low modulus of elasticity material in order to reduce stress concentration at the critical neck location placed on the bone cement or bone by the much stronger metal prosthesis. Preferably, ultra high molecular weight polyethylene or similar materials are used to form the collar and which are not bonded to the stem. The bone cement contacting stem of the prosthesis is held firmly within the intramedullary canal of the bone by the collar and methyl methacrylete bone cement which is used as a grouting agent. The stresses ordinarily imposed upon the calcar and the medial wall of the bone at the highly stressed attachment point are minimized because of the impact and force absorptive characteristics of the low modulus of the elasticity of the plastic component used in the collar and which has the necessary rigidity combined with flexibility and resiliency to absorb shocks as well as stresses which are normally imposed upon the bone structure. Apparently, the separate collar, which may be circular or U-shaped is operably positioned during surgery and while the bone cement is still in the viscous stage.
A number of the recognized problems relating to the use of bone cement have been addressed in three patents to Raab. U.S. Pat. No. 4,281,420 propose to enhance the bone cement--prosthesis interface by treating the entire boundary layer or bone cement contacting surface of the prosthesis stem with a thin layer of bone cement cured under optimum conditions. The annealed and cured film interacts with the ambient cured bone cement used during the surgical implant technique to enhance the interface strength. The preferable film thickness is stated to be about 0.002 inch, but may be much less. The related Raab patents incorporated herein are U.S. Pat. Nos. 4,336,618 and 4,365,359, and the chapter entitled "Mechanical Properties of Bone Cements In Vitro and In Vivo " identified as a reference in U.S. Pat. No. 4,365,359, is also incorporated by reference herein.
The three Raab patents involved the use of a metal prosthesis having a polymer precoated stem portion which is inserted into the interior of the bone. The polymer precoat is or is compatible with bone cement which is stated to comprise a mixture of polymethyl methacrylate (commonly referred to as PMMA polymer and methyl methacrylate monomer) and optionally including a styrene co-polymer of PMMA. After the surgeon has formed the desired opening in the bone and the prosthesis is ready to be affixed, the bone cement is prepared in the operating room. Each batch of bone cement mixed has slightly different final characteristics due to the conditions under which that batch is prepared. The dough like mixed bone cement is placed in the opening by the surgeon and the prosthesis properly positioned relative to the bone by the surgeon. The bone cement is then allowed to cure in place. The PMMA cement is typically applied by the surgeon in a highly viscous doughy state with the potential for an inadequate degree of contact between the implant and the bone cement.
U.S. Pat. No. 4,302,855 to Swanson is entitled "Plug for the Intramedullary Canal of a Bone and Method". The disclosed bone plug is formed to control or restrict the flow of bone cement used to affix a prosthesis to a bone. The resilient plug body is configured or formed in a frusto-conical shape and is surgically inserted into the intramedullary canal a sufficient distance to avoid contact with the prosthesis stem while forming a flow plug in the canal. When the prothesis stem is being fixed to the human bone, the plug serves to block escape or passage of bone cement through the intramedullary canal from adjacent the prosthesis stem during both insertion and curing of the prosthesis. If the plug operates perfectly, and the surgeon has no way of verifying or correcting for improper plug operation during surgery, several desired goals are accomplished. First, by blocking the intramedullary canal, body fluid contamination or dilution of the bone cement during curing is reduced. The second purpose is to permit the bone cement to be worked in the canal under pressure where it be forced into the bone intertices to enhance mechanical strength and minimize voids in the bone cement formed by trapped air. Under pressure, the dough like consistency of the bone cement can thereby achieve a better mechanical configuration of fit to the bone structure and bond to the prosthesis.
U.S. Pat. No. 4,283,799 to Pratt, et al is entitled "Pre-coated Body Implant". The disclosed metal joint prosthesis is provided with a stem portion that attaches to or is inserted into a surgically formed opening in a skeletal member of a human body. The bone cement contacting surface portion of the stem is precoated with a layer (preferably three to five millimeters in thickness) to increase the strength of the prothesis--bone cement interface. Due to the substantial difference in the Young's or elastic modulus (stress vs. strain) of the hardened cement and the metal prosthesis such relative strength become critical and the gradual transition is highly desirable. The preformed coating also serves the purpose of spacing the prosthesis stem from the bone to avoid potentially damaging direct contact therebetween and assures an optimal minimum thickness.
Conventional engineering and processing procedures can be employed to produce a layer of prebonded cement with the desired mechanical properties and devoid of trapped air bubbles or other defects prior to the surgical procedures. By gradually tapering the elastic properties from the implant into the cement and on in fact to the surrounding bone produces a system which would have a minimal relative motion or strain between the different surfaces and would enhance the interface bonding.
Pratt further discloses that an optimum thickness precoating of a metal prosthesis insures there will be no direct prosthesis or metal to bone contact in the event of improper positioning. That is, the prosthesis will be assured at least some interface cement covering and the covering will be of optimal minimum essential thickness at the critical or high stress transfer points. Also, by increasing the mass of the prosthesis with the polymerized cement layer, better compaction of the doughy cement into the intertices of cancellous bones can be achieved. The larger stem tends to act as a plunger during insertion for forcing the doughy cement under pressure into the cancellous bone.
As mentioned previously, the bone cement is conventionally batch mixed in the operating room as required and each batch may vary from optimum strength. During preparation of the dough like mixture, substantial quantities of air are easily entrapped. Such entrapped air remains as a porosity in the cured cement mantle and greatly reduces its strength. By applying a pressure or exerting compressive forces on the cement the porosity may be substantially reduced through reduction of the total volume of pores and the size of the cement voids which remain.
At the time of the Pratt patent, only two bone cements were approved for surgical use by the Federal Drug Administration although any others should be considered within the scope of the present invention. The two approved compositions were both two component systems which are mixed and kneaded until a doughy consistency is obtained. Immediately after mixing, the doughy cement is forced into the bone cavity which has been surgically prepared to receive the prosthesis. The bone cement is forced into the cavity with adequate pressure to place the doughy mixture into the intertices of the bone to provide a good physical interlock by the bone cement after curing. Subsequent to placement of the doughy mixture within the cavity, the prosthesis is then inserted or properly located within the bone cavity. In the case of the prosthesis of Pratt, the precoated surface may be treated during surgery with a suitable monomer or chemical to make the surface tacky or dissolve some of the surface of the polymer precoat prior to insertion and thus foster a good interfacial bond between the preformed bone cement and the new bone cement.